Continuous processes for the production of condensates using phosgene (for example, the production of aromatic polycarbonates or polyester carbonates or oligomers thereof) by the two-phase interfacial method generally have the disadvantage that more phosgene than is necessary for the product balance has to be used in order to accelerate the reaction and/or to improve phase separation. The excess phosgene is then degraded in the synthesis in the form of secondary products, for example additional sodium chloride or alkali metal carbonate compounds.
For example, a phosgene excess--based on the diphenolate used--of 20 to 60 mol-% is used in the continuous two-phase interfacial process for the conventional production of aromatic polycarbonates.
Thus, DOS 2,305,144 describes a process for the continuous production of polycarbonates, in which the two reactive phases are combined in the presence of amines in a mixing zone under essentially oil-in-water emulsion conditions and the phosgenation step takes place in a reaction zone after mixing. Specially designed flow arrangements are intended to ensure that the volume-time yield of the reaction is increased. The disadvantage lies in the large quantity of aqueous phase which promotes secondary phosgene reactions.
According to DOS 2,353,939, the properties of a polycarbonate produced by the two-phase interfacial method are said to be improved by controlling the reaction through adjustment of the pH value. The disadvantage lies in the excess phosgene used, in addition to which the process is not continuous.
According to the teaching of EP 0,282,546 chlorocarbonate terminated condensates are said to be obtained with high phosgene yields by a version of the two-phase interfacial process in which a stable diphenol/water/sodium hydroxide suspension and phosgene are continuously introduced at the same time into an organic phase and the reaction product is subsequently isolated. pH values of 2 to 5 are maintained during the reaction. Disadvantages arise out of the technical problems involved in dosing the suspension and the low pH value which considerably increases the phosgenation time.
According to EP 0,263,432, chloroformyl-terminated condensates or polycarbonates can be produced from aqueous diphenolate solution and organic solution by introducing phosgene into a heterogeneous mixture at pH values of 8 to 11, at temperatures of 15 to 50.degree. C. and with a phosgene excess of at least 10 mol-% and continuing the phosgenation reaction with simultaneous introduction of alkali metal or alkaline earth metal hydroxides.
According to DOS 2,725,967, it is favorable to the phosgene yield of a continuous process if the aqueous phase and the organic phase containing dissolved phosgene are initially combined in a tube and subsequently introduced into a reactor of the tank type. The residence time in this tube should be between 0.5 and 15 seconds. The phosgene excess of the reaction is at least 10 mol-%. The disadvantage lies in the fact that the phosgene excess is still extremely high. In addition, the process is attended by the disadvantage that the phosgenation reaction takes place at unfavorable phase ratios (oil-to-water=0.2 to 1) so that separation of the two phases on completion of the reaction is definitely possible.
According to EP 0,306,838-A2, the phosgenation reaction is monitored in situ using an automatic chlorine detector. Carrying out the process in this way suppresses variations in the chemism of the reaction and is said distinctly to improve the technical properties of the polycarbonates. The concept on which the process is based lies in the recycling of unreacted diphenolate to the process. However, the process is attended by the disadvantage of secondary phosgene reactions which even take place during recycling.
According to EP-0,339,503-A2, the secondary phosgene reactions are said to be increased by the presence of a high initial sodium hydroxide concentration. According to this patent, therefore, the diphenol/sodium hydroxide/water solution is combined with the organic phases in an alkali/hydroxy ratio of less than 2:1 (less than equivalent quantity of alkali metal hydroxide), oligomers having a molecular weight of 300 to 3,000 g/mol being formed in this first reaction stage. The water-to-oil phase ratios are greater than 1. In addition, the secondary phosgene reactions are still extremely unfavorable.
According to EP-0,305,691-A2, a fine emulsion produced by intensive mixing is favorable to the reaction in the two-phase interfacial process, albeit with a very large excess of phosgene (20 to 100 mol-% excess). The large quantity of phosgene promotes good phase separation despite intensive mixing of the emulsion at the beginning of the reaction. However, the phosgene yield is extremely unfavorable.
Accordingly, the continuous production of condensates or polycondensates by the two-phase interfacial method has hitherto only been possible with a large excess of phosgene or in unfavorable phase ratios (use of water) or with phase separation problems.
According to non prepublished U.S. Pat. No. 5,043,203 polycarbonate is also prepared by a process comprising several steps. In contrast to the procedure according to instant invention there is no separate formation of a bisphenolate-solution at the beginning of the process. The corresponding European equivalent to the U.S. Pat. No. 5,043,203 has the number 0456052.